Plastic materials have continued to make significant advancements into the packaging industry due to the design flexibility of their material and their ability to be fabricated in various sizes and shapes commonly used in the packaging industry. The deployment of plastic materials into films, trays, bottles, cups, bowls, coatings and liners is already commonplace in the packaging industry. Although plastic materials offer the packaging industry many benefits with an unlimited degree of design flexibility, the utility of plastic materials has remained inhibited in situations where barrier properties to atmospheric gases (primarily oxygen) are necessary to assure adequate product shelf life. When compared to traditional packaging materials such as glass and steel, plastics offer inferior barrier properties which limits their acceptability for use in packaging items that are sensitive to atmospheric gases, particularly when the exposure to the atmospheric gases will entail extended time periods. The packaging industry continues to seek packaging materials which offer the design flexibility of plastics with the inherent recycle advantage of plastics and at the same time have the barrier properties of glass and steel.
The packaging industry has developed technology to improve the barrier properties of plastic containers by developing multi-layer containers that offer mixed polymer layers. These laminated packaging containers offer improved barrier properties approaching, but not comparable to, those of glass and steel while sacrificing many of the recycling benefits associated with single layer containers such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) bottles. Furthermore, depending on the mixtures of polymers, copolymers, blends, etc., used in the layers, clarity of the layered container is often substantially diminished. Maintaining the proper balance of recyclability, barrier properties, and clarity is most critical in bottling applications. However, these are common concerns among a wide range of plastic packaging uses.
PET has made significant inroads into bottling and packaging applications at the expense of the use of glass containers but primarily in applications where the needs for barrier properties are modest. A significant example is the use of PET for soft drink bottles. However, PET barrier properties have limited its use in the packaging of oxygen sensitive drinks such as fruit juices and beer. The most common size for PET soft drink bottles is the two liter capacity bottle but one liter and three liter bottles are also frequently seen. The wall thickness of PET employed for these larger sized bottles provides an adequate oxygen barrier for such products. Bottling fruit juice and other products of similar oxygen sensitivity in large bottles with thick PET walls has recently been commercialized. The increased wall thickness is needed to improve the barrier properties of the container but has a negative impact on the economics of the container. The ratio of packaging material to package volume has limited PET bottles to multi-serve container uses for packaging of oxygen sensitive foods and beverages. As the oxygen sensitivity of the packaged product increases or as the size of the package decreases, at some point the ratio of packaging material versus package volume becomes prohibitive. When that occurs, the production and use of thick walled conventional PET bottles is no longer economically viable as the cost of the packaging is disproportionate to the value of the packaged product. The availability of beverages and food in single serve plastic bottles and packages is an important economic consideration particularly for use in unusual sales locations such as at special events, in stadiums or arenas, and in similar situations where the amount of product sold is often determined by how quickly the product can be transferred from a multi-serve container to a single/consumer serving. Often, the sale of beverages in single serve glass or metal containers is prohibited at such locations because of the possibility that the empty containers may be hurled as missiles by rowdy attendees at such events. Sale of beverages in single serve plastic bottles, however, is normally permitted in all situations.
One possibility for extending the economic viability of packaging oxygen sensitive materials in smaller or individual serving containers is to decrease the thickness of the bottle wall so as to maintain the same proportion of packaging material to package volume as would be found for larger bottles. However, containers with thinner walls made from conventional bottling polyester permit more passage of oxygen to the packaged product than thick bottle walls. As such, the shelf life and other required features of the bottle would not be satisfactory. However, modified thin bottle walls which maintain or improve the oxygen barrier characteristics of conventional bottling polyester could provide an answer. The use of multi-layer bottles that contain an inner, sometimes sandwiched, layer of a second higher barrier polymer material compared to the outer polymer layers, is already commonplace. Typically the center layer is a high barrier polymer that exhibits barrier properties by slowing the permeability of oxygen through the container wall. Such a system would be categorized as a passive barrier. A common construction for such passive barriers would comprise inner and outer layers of PET with a center layer of ethylene-vinyl alcohol (EVOH) polymer. Another method for providing increased oxygen barrier properties is the incorporation into the bottle walls of substances capable of intercepting and scavenging oxygen as it attempts to pass through the walls of the container. This method also affords the opportunity to eliminate unwanted oxygen from the package cavity wherein said oxygen may have been inadvertently introduced during packaging or filling. This method of providing oxygen barrier properties where a substance consumes or reacts with the oxygen is known as an "active oxygen barrier" and is a different concept from passive oxygen barriers which attempt to hermetically seal a product away from oxygen via the passive approach.
One method for use of active barriers would be to make a three layer bottle which actually appears to be a mono-layer bottle. In the three layer bottle, the inner and outer layers are made of the same generic family of polymeric materials. The method applies to many packaging articles, but in the case of a bottle, the construction would comprise two polyester layers sandwiching a middle layer having outstanding oxygen scavenging characteristics atypical of the outer polyester layers. When the middle layer is very similar to the outer polyester layers, the article appears to be only a single layer. Of course many options exist including the use of a relatively homogeneous mono-layer comprising oxygen scavenging copolymers.
Incorporation of an active oxygen scavenger into the walls of a bottle provides a very effective means for elimination or at least control of the amount of oxygen which reaches the cavity of the package. However, there are some exacting demands which are placed upon the active oxygen scavenging walls of the bottle. One consideration is that the relatively thin walls of the bottle should be of sufficient strength and rigidity to withstand the rigors of filling, shipping, and use by consumers. The oxygen scavenging capacity of the bottle walls should be of sufficient capacity to allow for adequate shelf life and normal product turnover intervals. Shelf life and turnover intervals require that the oxygen scavenging should occur for extended periods of time. Most packaged products are stored and transported at room temperature or under refrigeration which mandates the necessity for oxygen scavenging activity at these temperatures. Of course, the oxygen scavenger should exist as a solid at these temperatures so as to be shaped and formed into packaging articles, i.e., these storage and transporting temperatures must be below the glass transition temperature (T.sub.g) of oxygen scavenging compositions. The preferred compositions would absorb oxygen at a rate faster than the permeability of oxygen through the packaging wall for the planned shelf-life of the packaged product while having enough capacity to remove oxygen from within the package cavity if necessary. In those applications requiring clarity, the packaging article should have optical properties approaching those of PET. Finally, the preferred thin walled bottles should be suitable for recycle with other polyester bottles. In order to be meaningful, the recycling must be conducted without the need for any special physical processing such as delamination or the need for any special chemical processing such as depolymerization. What is needed are oxygen scavenging compositions, methods for the production of said compositions and methods of using the compositions in packaging articles so as to satisfy all the demands as recited above.